In the production of cast iron workpieces, i.e. castings from cast iron, and especially relatively large castings, it is very difficult to have a perfect workpiece without further finish welding or repair welding after the casting process. Examples of such large castings include hubs for modern wind turbines, which may weigh more than 10 tons and have a diameter of more than 5 meters and which are usually made of ferrite-based spheroidal graphite cast iron having both good ductility and good fatigue strength. Shrink holes appearing upon removing or cutting the risers or runners are the most frequent defects found in the spheroidal graphite cast iron workpieces. There are many causes of the defects. For example, the cooling down of risers and runners is much quicker than of the bulky workpiece, and thus causes insufficient feeding; and expansion of the sand mould may cause insufficient molten iron therein etc. Further defects include gas porosity and dross etc. All these defects occur particularly in thick-walled workpieces.
The structural requirement for such large castings are usually very critical due to the strong loads they need to withstand, since any small defect in cast iron may reduce the fatigue strength and cause its failure earlier than designed lifespan.
For example, in modern wind industry, many of structural components utilized in wind turbines are thick-walled workpieces, the most detectable defects such as shrink holes and dross often come up unexpectedly, and generally leading to the rejection of these workpieces.
Unfortunately, cast iron is less weldable than cast steel because it contains more carbon than steel, which will enter melting pool and increase carbon content in the melts and produce unwanted carbide during subsequent solidification. The traditional repair welding process for steel is therefore not available to cast iron. Extreme care and special skill are required while welding cast iron workpieces because the welds are prone to crack.
During conventional casting process, the carbon dissolves in the molten iron at high temperature and combines with ferrite at low temperature. Since the molten iron cools down and solidifies slowly in sand mould, there is enough time for stable phase transformation. The product of phase transformation is normally the ferrite or pearlite base distributed with the graphite. However, during a welding process, even though the workpieces can be preheated to high temperature before welding or by use of a heat isolator, it is still difficult to retard the rapid solidification of the melts in melting pool and give enough time for adequate graphite segregation. This results the formation of carbide or martensite and hence reduces the ductility of welds drastically. The size of melting pool is too small to retard the rapid solidification of melts.
Current repair welding is problematic when being applied on cast iron workpieces due to the potential stress caused and potential cracks at the welds. Post-welding heat treatment, which is normally required to relieve stress and improve ductility, is usually unfeasible for large castings such as the above mentioned wind turbine hub. This results in a high cost of cast iron workpieces since a significant percentage of them cannot be used due to casting defects.